PROJECT SUMMARY The ability of contracting skeletal muscle to modulate sympathetic vasoconstriction (?functional sympatholysis?) is essential to ensure adequate blood flow and oxygen delivery to active muscle in the face of elevated sympathetic outflow during exercise. With advancing age, impaired functional sympatholysis, together with elevated sympathetic nerve activity, contributes to malperfusion of contracting skeletal muscle. Thus, elucidating the signaling mechanisms that underlie the ability of active muscle to modulate sympathetic vasoconstriction will advance our understanding of the basic mechanisms of blood flow regulation in humans and lay the foundation for new strategies to improve peripheral blood flow and exercise tolerance in aging and clinical populations. Accordingly, the overall goal of the proposed research is to investigate endogenous acetylcholine (ACh) as a novel mechanism of sympatholysis and determine whether a reduced contribution of ACh explains impairments in functional sympatholysis in older adults. Specific Aim 1 will test the hypothesis that endogenous ACh limits sympathetic vasoconstriction in contracting skeletal muscle in young, healthy humans. To test this, forearm blood flow (Doppler ultrasound) and arterial pressure (brachial artery catheter) will be measured to assess vasoconstriction in response to local, intra-arterial infusion of phenylephrine (?1- adrenergic receptor agonist) at rest and during rhythmic forearm exercise under control conditions and during local inhibition of muscarinic ACh receptors via atropine. Specific Aim 2 will test the hypothesis that the contribution of ACh to functional sympatholysis is reduced with advancing age. This will be achieved by comparing the effect of atropine on sympathetic vasoconstriction during exercise in older adults to that in young adults. Because exercise training improves vascular sensitivity to ACh, Specific Aim 3 will test the hypothesis that a four week handgrip training intervention will improve functional sympatholysis in older adults through an increased contribution of ACh, and further, that this will be associated with improvements in functional outcomes including maximal forearm work rate and time to fatigue during a submaximal workload. The expected outcomes will provide novel insight to the basic physiology underlying vascular control in humans. Moreover, the findings will provide mechanistic insight to the age-related decline in the ability of contracting skeletal muscle to blunt sympathetic vasoconstriction, and will assess the therapeutic utility of an exercise intervention to improve functional outcomes in older individuals. This research is relevant to NHLBI?s mission and the research priorities of the Vascular Biology and Hypertension Branch, as impaired vascular control with advancing age leads to a decline in functional capacity and quality of life and increases the risk of ischemic cardiovascular disease.